The invention lies in the field of measurement technology and relates to a tracking method and to a measurement system with a laser tracker, according to the preambles of the respective patent claims. The tracking method serves for the automatic tracking of a target point, in particular a moving target point, with the aid of the measurement beam of a laser tracker. The measurement system with a laser tracker is designed for carrying out the method.
So-called laser trackers are widely used for the measurement of the position of moving target points. A laser tracker is understood to be a device comprising at least one distance meter functioning with a focused laser beam (called measurement beam in the subsequent description). In such a laser tracker, for example, the direction of the measurement beam towards the target point is set with the help of a mirror rotatable about two axes, and is detected by an angle transmitter assigned to each one of the rotation axes. The target point to be measured is provided with a retro-reflector (in particular cube-corner prism or arrangement of three mirrors which are perpendicular to one another), wherein the retro-reflector reflects the incident measurement beam of the laser tracker, back to the tracker. Therein, the reflected measurement beam runs coaxially to the emitted measurement beam if the measured beam is incident on the reflector in an exactly centric manner, and parallel to the emitted beam if the latter impinges the reflector non-centrically. Depending on the type of the tracker, an absolute distance between the laser tracker and the target point and/or a change of this distance is deduced from a comparison of the emitted and reflected laser light. The position of the reflector or the target point relative to the tracker is computed from the angles detected by the angle transmitters, and the distance detected by the distance meter.
A part of the reflected measurement beam is usually guided onto a PSD (position sensitive device). The parallel displacement of the reflected measurement beam relative to the emitted measurement beam is deduced from the position in which the reflected measurement beam is incident on the light-sensitive surface of the PSD. Corresponding measurement data define the parallel offset of the reflected measurement beam, and are used for controlling the measurement beam direction, in a manner such that the measurement beam follows the target point (tracking) when it moves. For achieving this, the parallel offset between the emitted and the reflected measurement beam is reduced in size or kept as small as possible, by changing the measurement beam direction or the alignment of the mirror which directs the measurement beam respectively.
It is evident that control of the measurement beam direction by way of the parallel offset between the emitted and the reflected measurement beams, includes a small, but not negligible delay, which limits the speed with which a target point may move and still be tracked. If the target point moves more quickly, the measurement beam moves off the reflector before its direction can be corrected accordingly, such interrupting the tracking process as well as the position measurement. The same may happen if an obstacle gets between the tracker and the target point, so that the measurement beam is interrupted. If the laser tracker or the measurement beam of the laser tracker respectively “loses” the reflector, the operating person is made aware of this and, given a suitable design of the tracker, may initiate a search routine.
As soon as the target point has been “found” again, which means that the measurement beam is again incident on the reflector and is reflected by the latter, measurement of the position of the target point and tracking thereof with the aid of the measurement beam can be taken up again, which however, may necessitate re-initiation of the distance measurement. The mentioned tracking interruptions get more frequent, if movements of the target point get less controlled, and if the diameters of reflector and measurement beam get smaller. At the beginning of a measurement process when the tracker is not yet aimed at the target point, the same conditions prevail as during the mentioned tracking interruptions.
In systems with automatically moved target points, the described tracking interruptions can be prevented simply by adapting the movements of the target point precisely to the tracking capabilities of the laser tracker. This, however, is significantly more difficult in measurement systems, in which the target point is moved by hand, i.e. where the target point or the object on which the target point is arranged, is moved by a person. In such systems, tracking interruptions cannot be avoided completely and may even occur relatively frequently. This is particularly the case for measurement systems in which the object carrying the reflector for example is a hand-held touch tool or a hand-held scanner, wherein the touch tool or scanner is guided over an object to be measured by a measuring person, and the position and orientation of the touch tool or scanner is tracked by a laser tracker, and is registered in an essentially continuous manner. In particular for such systems, it would be desirable to be able to bridge tracking interruptions rapidly and in an automatic manner, that is to say in particular without necessitating any intervention by the operator.
It is also known to provide laser trackers with an overview camera having an as large as possible viewing angle (for example ±20° in all directions), the overview camera being aligned with the tracker in a manner such that the measurement beam can be directed to a target point recognised on the camera image. Aiming of the measurement beam towards the target point is initiated by an operating person observing the camera image and suitably indicating the image region in which the target point is imaged.